Apparatus for coating paper



Dec. 20, 1966 P. B. CRESSEY, JR., EI'AL 3,292,577

APPARATUS FOR comma PAPER Filed Feb. 5, 1964 FIGZ iii 14 I r J a INVENTORS PHILO B. CRESSEY JR EBEN w. FREEMAN 20 i .FM,6ZZ% fiZfa/d ATTORNEYS United States Patent APPARATUS FOR COATING PAPER Philo B. Cressey, Jr., Gorham, and Eben W. Freeman, Portland, Maine, assignors to S. D. Warren Company, Boston, Mass., a corporation of Massachusetts Filed Feb. 5, 1964, Ser. No. 342,652 8 Claims. (Cl. 118603) This invention relates to apparatus for coating paper and more particularly to an air knife coating machine in which a doctor blade is employed in combination with the coating roll to prevent air bubbles in the liquid coating from being applied to the paper web.

' Many coated papers, and especially papers having a socalled cast coated surface, undergo a coating process designed to impart a high gloss and extreme smoothness to the paper surface. However, the coated surface frequently contains microscopic holes and pits which detract from the appearance of the paper and interfere with subsequent printing operations.

These'microscopic defects are caused by minute gas bubbles which have been entrapped in the coating. The bubbles, containing principally air, are created and develop in the coating composition principally from one source, the interaction of the coating roll with the coating pool. The coating roll, as it revolves in the pan of coating composition, draws air into the coating pool which forms into a multitude of entrapped air bubbles which tend to congregate at the surface.

In the past, the coating composition, containing these entrapped air bubbles, has been applied by the coating roll to the underside of the paper web. The bubbles burst when drying heat is applied to the coated web causing tiny holes and pits to appear in the coated surface.

So-called foam killers and anti-foam agents have been used in coating compositions to retard the development of gas bubbles and are relatively efiective in some coating applications. But for cast-coated papers, their action falls short of completely eliminating air bubbles to the extent required to achieve a coating having the desired glossiness, smoothness and absence of holes and pits. Furthermore, anti-foam agents are costly and sometimes produce other defects in the coated paper.

I We have found that mounting a doctor blade so that it presses firmly against a submerged portion of a smooth surfaced coating roll will wipe the clinging air bubble layer from the roll surface at that point. By arranging the location of the pump inlet and outlet spillways and by carefully positioning the doctor blade, a current flow can be set up whereby virtually all the air bubbles are confined to the side of the coating pan opposite the coating application side and are continuously discharged from the pan.

Accordingly, it is an object of this invention to provide an apparatus for coating paper which is simple, easy to manufacture and install, yet is extremely effective in producing a coated paper that is virtually free from pits and holes.

It is another object of this invention to provide an apparatus for coating paper'which is effective in preventing entrapped air bubbles from being deposited on the web coating so that the use of anti-foam agents can be dispensed with or significantly reduced.

Other objects and advantages of this invention will become apparent from a study of the accompanying drawings and descriptive matter in which is illustrated and described a preferred embodiment of the invention.

In the drawings:

FIG. 1 is a diagrammatic elevational view showing the air knife coating machine incorporating the invention; and

3,292,577 Patented Dec. 20, 1966 "ice FIG. 2 is an enlarged diagrammatic view of the coating application portion of the machine shown in FIG. 1.

Referring now more particularly to the drawings, the air knife coating machine, as shown in FIG. 1, has a machine tank 10 in which the coating composition is temporarily stored. Machine tank 10 normally has a 30 to 50 gallon capacity and can optionally be supplied by a larger storage tank (not shown) by means of a float control actuated pumping system.

A coating pan 12, also called a color pan, has side walls 14 and a bottom wall 16. An inlet orifice 18 is formed in the center of bottom wall 16 and is fitted with a feed pipe 20 leading from machine tank 10. A pump 22 and filter 24 are mounted in feed pipe 20 to continuously draw clean liquid coating composition into coating pan 12. The coating composition completely fills ,pan 12 and overflows both side walls 14 into spillways 26 which channel the coating composition into common return pipe 28. Return pipe 28 empties against baffle 30 from which the coating drains into machine tank 10. In this process, the coating is continuously circulated through filter 24 where impurities are removed and irregularities in consistency are strained to ensure uniformity of composition.

Positioned over pan 12 is a frame mounted coating roll 32 normally having a 6 to 12 inch diameter. The roll length is selected in accordance with the width of in a frame (not shown) and can be driven at various rotational speeds to attain the desired roll surface speed. A the paper wed to be coated. Coating roll 32 is journalled well-formed paper web 34, of the type commonly used as a base stock for the production of high quality paper, passes from a feed roll (not shown) over coating roll 32 around breast roll 36 and usually into a drying unit (not shown). The web 34 can, of course, be led through various types of operating stations both before and after its passage through the air knife coating machine, but these operations form no part of the present invention.

Coating roll 32 is horizontally mounted for rotation and positioned over coating pan 12 so that its lower peripheral surface is immersed in the coating compositing which fills the pan. That is, the roll surface 38 depends below the upper edge 40 of the coating pan side walls 14. As the coating roll revolves, a film of coating composition is picked up from the coating pan 12 by the roll surface 38 and is applied to the underside of paper web 34 as the web passes over coating roll 32. A metering bar 42 limits the amount of coating carried by the roll to the underside of the paper Web 34. The coating roll surface should be extremely smooth and is preferably made from stainless steel or chrome plate.

After the coating has been applied to the web, the web moves to breast roll 36 where the coating is precisely spread and distributed by an air knife 44 which throws a thin, flat stream of air from a slot 46 in its metal blade body 48. The slot 46 extends across the web. The air stream smooths the surface and removes excess coating from the web which falls into spray pan 50 beneath deflector 52. An outlet orifice 54 is formed in the bottom wall 56 of spray pan 50 and connects with return pipe 58 which discharges the collected coating composition against baffie 60 into machine tank 10. The paper web is then dried and finished by subsequent operations.

Looking now at FIG. 2, the doctor blade feature of this invention will be described in detail. A doctor blade 62 is mounted on the machine frame or otherwise in a manner such that it is positioned in the coating pan 12 beneath the liquid coating level. The doctor blade, preferably made from bronze or heavy duty plastic to avoid scoring the roll, has a blade edge 64, a blade body 66 and a curved deflector portion 70. The blade is preferably spring tensioned to fit closely against the coating roll surface 38 for the purpose of wiping clean the roll surface at the line of contact therebetween. The blade preferably extends completely across the coating roll 32 so as to be wider than the paper web to be coated.

' An important feature of this invention is the arrangement of the mechanical elements whereby two compartments or regions are defined within the pool of coating composition. The doctor blade is preferably mounted so that its blade edge 64 contacts the roll surface 38 and forms an angle of approximately 45 degrees with a tangential line drawn in the direction of coating roll rotation. This angle can be varied, but preferably should be in the range of 30 to 60 degrees. The blade body 66 extends almost to the bottom wall 16 of coating pan 12 so as to divide the pan into two compartments with just enough space remaining between the lowermost portion of the blade body 66 and the bottom wall 16 to allow the coating liquid to pass between the compartments. An alternate arrangement would be to extend the blade body to bottom wall 16 and to provide passageways through the blade body.

It will now be seen that air drawn into the coating composition by the roll at entry region 68 develops into bubbles which are carried along by the roller surface 38 until they reach blade edge 64 at which point they are separated from the roll and move downwardly along the lower side of blade body 66. The bubbles are then downwardly directed by a curved deflector member 70 to the area adjacent pump inlet opening 18. Inlet opening 18 is positioned to direct the pumped-in coating composition upward or slightly to the left (looking at FIG. 2). The pumping action forces the air bubbles that have been deflected by deflector member 70 in an upward flow towards the surface where they spill over upper edge 40 of the left coating pan side wall 14. The air bubbles and foam are returned to the machine tank where they are skimmed off and diverted to a waste receptacle. Thus, on the roll entry side or compartment, 21 circulatory current is established by the interaction of the roll surface movement and the pump inlet current whereby air bubbles are carried along the roll surface, are separated by the doctor blade, are directed to the surface by the pump flow and are discharged into the spillway.

The second so-called compartment or region is on the side of the coating pan where the roll leaves the coating pool carrying coating to the web. This side is relatively placid and free from currents except for the slight turbulence caused at the exit region 72. Bubbles caused by this turbulence flow away from the roll along the coating pool surface and overflow into spillway 26. Therefore, the air bubble incidence is low in this region and the coating composition picked up by roll surface 38 is relatively bubble-free.

In operation, a paper web 34 is carried by coating roll 32 and breast roll 36. Coating roll 32 applies coating composition, picked up from the coating pool in pan 12, to the underside of the paper web. An air knife 44 thereafter distributes the coating on the web surface. Coating is continuously pumped from tank 10 to the coating pan 12, overflows the pan and returns to tank 10. Excess coating blown from the Web by air knife 44 also returns to tank 10.

A doctor blade 62 is pressed firmly against the underside of the submerged coating roll 32 to wipe the roll and is mounted so as to divide the coating pan 12 into two compartments. Air which becomes bubbles is drawn into the coating pool primarily by the roll movement. A circulatory path in the coating pool is created in the first compartment along the roll surface, doctor blade, pan side walls and over the spillway. By this means, the air bubbles are effectively confined to the first or roll entry compartment. The second compartment, on the exit side of the roll, is calm and permits the coating applied by the roll surfacc 33 9 be essentially bubble-free.

Obviously, many modifications and variations of the present invention are possible in light of the above teachings. Therefore, it is to be understood that the invention is not limited in its application to the construction and arangement of parts specifically described or illustrated, but is to be limited only by the scope of the appended claims and their equivalents.

We claim:

1. Apparatus for coating paper comprising:

(a) a coating pan;

(b) means for continuously delivering liquid coating into said pan at a rate producing a substantially uniform liquid coating level in said pan and a continuous overflow of liquid coating from said pan;

(c) a coating roll disposed above said pan and mounted for rotation about a horizontal axis, said roll carrying a paper web on its upper periphery and moving through the pan-contained coating pool at its lower periphery to apply liquid coating to the underside of the paper web, said roll producing a roll periphery-following current in the coating pool as a result of relative motion therebetween, said roll peripheryfollowing current containing entrapped gas bubbles; and

(d) means mounted within said coating pan including a doctor blade pressed against said roll below ,the substantially uniform liquid coating level and remaining against said roll during rotation of said roll toward said blade for wiping clinging gas bubbles from the lower periphery of said roll and for directing the bubbles away from the exit side of the roll into the continuous overflow of liquid coatingfrom said pan. I

2. Apparatus of claim 1 wherein said liquid coating delivery means includes a coating storage tank, and inlet formed in a wall of said coating pan, a feed pipe connected at one end to said coating storage tank and at its other end to said pan inlet, a filter mounted in said feed pipe, and a pump mounted in said feed pipe for pumping liquid coating from said storage tank, through said filter, and into said coating pan.

3. Apparatus of claim 1 wherein said last mentioned means includes a partition member positioned transversely between the pan bottom and side walls and the lower periphery of said roll to divide the pan into a roll entry compartment and a roll exit compartment, and passage means between the two compartments for permitting the coating levels in the two compartments to remain equalized.

4. Apparatus of claim 3 wherein said doctor blade is mounted on the upper margin of said partition member and contacts the lower periphery of said coating roll.

5. Apparatus of claim 3 wherein said passage means is formed in said partition member.

6. Apparatus of claim 3 wherein said passage means is spaced from said partition member.

7. The apparatus of claim 1 wherein said doctor blade contacts the lower periphery of said coating roll at an angle in the range of 30 to 60 degrees from a tangential line drawn in the direction of roll rotation.

8. The apparatus of claim 1 wherein said coating roll has a smooth surface.

References Cited by the Examiner UNITED STATES PATENTS 366,186 7/1887 Richards. 1,255,245 2/ 1918 Taylor. 2,649,758 8/ 1953 Cowgill 118-602 2,911,320 11/1959 Phillips 1l7111.X 3,196,038 7/ 1965 Schoch et al. l1863 X CHARLES A. WILLMUTH, Primary Examiner.

L. G. MACHLIN, Assistant Examiner. v

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No 5 292 5 7 December 20, 1966 Philo Bu Cressey, Jr. et al It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 2 lines 28 to 30 strike out "in a frame (not shown) and can be driven at various rotational speeds to attain the deslred roll surface speed. A the paper wed to be coated. Coating roll 32 is journalled"; and insert instead the paper web to be coated Coating roll 32 is journalled in a frame (not shown) and can be driven at varoius rotational speed: to attain the desired roll surface speed. A line 41, for "compositing" read composition Signed and sealed this 26th day of September 1967.

(SEAL) Attest:

ERNEST W. SWTDER EDWARD J. BRENNER Attesting Officer Commissioner of Patents 

1. APPARATUS FOR COATING PAPER COMPRISING: (A) A COATING PAN; (B) MEANS FOR CONTINUOUSLY DELIVERING LIQUID COATING INTO SAID PAN AT A RATE PRODUCING A SUBSTANTIALLY UNIFORM LIQUID COATING LEVEL IN SAID PAN AND A CONTINUOUS OVERFLOW OF LIQUID COATING FROM SAID PAN; (C) A COATING ROLL DISPOSED ABOVE SAID PAN AND MOUNTED FOR ROTATION ABOUT A HORIZONTAL AXIS, SAID ROLL CARRYING A PAPER WEB ON ITS UPPER PERIPHERY AND MOVING THROUGH THE PAN-CONTAINED COATING POOL AT ITS LOWER PERIPHERY TO APPLY LIQUID COATING TO THE UNDERSIDE OF THE PAPER WEB, SAID ROLL PRODUCING A ROLL PERIPHERY-FOLLOWING CURRENT IN THE COATING POOL AS A RESULT OF RELATIVE MOTION THEREBETWEEN, SAID ROLL PERIPHERYFOLLOWING CURRENT CONTAINING ENTRAPPED GAS BUBBLES; AND (D) MEANS MOUNTED WITHIN SAID COATING PAN INCLUDING A DOCTOR BLADE PRESSED AGAINST SAID ROLL BELOW THE 